1. Field of the Invention
The present invention relates to an image forming apparatus including a motor configured to rotate a rotary polygon mirror.
2. Description of the Related Art
In an image forming apparatus such as a copying machine or a printer, as a unit configured to form an electrostatic latent image on a photosensitive member, there is hitherto widely used a light scanning apparatus for scanning the photosensitive member with a light beam which is emitted from a semiconductor laser and deflected by a rotary polygon mirror.
It is known that a rotary polygon mirror driving motor (hereinafter simply referred to as a motor) configured to rotate a rotary polygon mirror has large inertia and takes a long period of time until a rotation speed thereof is stabilized. Further, along with the recent increase in operation speed of the image forming apparatus, it becomes more necessary to rotate the motor at extremely high speed when an image is formed, and thus, a rise time necessary for the motor to reach a main rotation speed when an image is formed from a stopped state becomes longer. Further, high speed rotation of the rotary polygon mirror presents a noise problem due to wind noise and a problem in which a life-time of the motor is shortened.
An image forming apparatus starts image formation in a state in which the rotation speed of the motor is stabilized. Therefore, generally, a first copy output time, that is, a time period necessary from when a copy start button is pressed down until a first paper sheet is output is affected by the rise time of the motor.
Japanese Patent Publication No. H07-36600 discloses that, when image data is transferred to a control device, the motor is rotated at a preliminary rotation speed which is lower than the main rotation speed of a time of image formation, and when a form feed signal is sent to the control device, the motor is rotated at the main rotation speed.
FIGS. 9A, 9B, and 9C are graphs showing characteristics of the motor. FIG. 9A shows a relationship between time and a rotation speed of the conventional motor. FIG. 9B shows a relationship between an ambient temperature of the motor and the rise time of the motor. FIG. 9C shows a relationship between a total operation time of the motor and the rise time of the motor.
In Japanese Patent Publication No. H07-36600, as shown in FIG. 9A, when the image data is transferred to the control device (hereinafter referred to as a pre-operation), rotation of the motor is started, and the motor is rotated at the preliminary rotation speed. When a receipt of a form feed signal (hereinafter referred to as a predetermined operation) is performed, the control device increases the rotation speed of the motor to rotate the motor at the main rotation speed for image formation. In this way, the rotation speed of the motor is controlled to be a single predetermined preliminary rotation speed during a pre-operation period from the pre-operation to the predetermined operation.
On the other hand, as can be understood from characteristics of the motor shown in FIG. 9B, the rise time of the motor from the stopped state of the motor till when the motor reaches the main rotation speed of 48,000 rpm is longer when the ambient temperature of the motor is low than when high. Further, as shown in FIG. 9C, the rise time of the motor is longer when the total operation time is long than when short.
The motor is required to become rotated with stability at the main rotation speed for image formation before latent image formation is started. Therefore, the preliminary rotation speed is set with reference to a case where the rise time of the motor is long, so that the motor becomes rotated with stability at the main rotation speed before the latent image formation is started. It follows that, when the rise time of the motor is short, the motor is rotated at the preliminary rotation speed for a time period longer than necessary, which reduces an effect of reducing noise during the pre-operation period and an effect of increasing a life-time of the motor.